Development of semiconductor devices having thin film transistors (TFTs) and thin film diodes (TFDs) formed on the same substrate and electronics systems including such semiconductor devices has been underway in recent years. As a method of manufacturing such a semiconductor device, a method of forming the semiconductor layers for the TFTs and the TFDs using the same crystalline semiconductor film formed on the substrate has been proposed.
Patent Document 1 discloses an image sensor having on the same substrate a photo sensor portion using the TFDs and a driver circuit using the TFTs on the same substrate. According to the Patent Document 1, the semiconductor layers for the TFT and the TFD are formed by crystallizing a non-crystalline semiconductor film formed on the substrate.
There are large cost advantages to forming the TFT and the TFD in a single unit on the same substrate, including a reduction in semiconductor device form factor and component counts. Furthermore, new products can be realized by adding new features which cannot be obtained by combining the conventional parts.
On the other hand, Patent Document 2 discloses the formation of the TFT using the crystalline silicon (crystalline silicon TFT) and the TFD using the non-crystalline silicon (non-crystalline silicon TFD) on the same substrate using the same semiconductor film (non-crystalline silicon film). Specifically, a catalytic element is added for promoting the crystallization of non-crystalline silicon only in the region of the non-crystalline silicon film formed on the substrate in which the active region of the TFT is to be formed. Then, a silicon film is formed such that only the region in which the active region of the TFT is to be formed is crystallized with an anneal process, and the region to be the TFD remains in the amorphous state. Using this silicon film, the crystalline silicon TFT and the non-crystalline silicon TFD can be easily formed on the same substrate.
Furthermore, a photo sensor TFT functioning as a photo sensor and a switching TFT functioning as a switching device are formed using the same semiconductor film (non-crystalline silicon film) in Patent Document 3. More specifically, the channel region of the photo sensor TFT is formed with a non-crystalline silicon film, while the source and drain regions and the active region of the switching TFT are formed with the crystalline silicon film. Furthermore, the non-crystalline silicon film for the channel region of the photo sensor TFT is made thicker than the crystalline silicon film of the active region of the switching TFT in order to enhance the photo sensor sensitivity.
The aforementioned TFTs are created separately using the method described below in Patent Document 3. First, a non-crystalline silicon film is formed on the substrate. Next, the non-crystalline silicon film is made partially thinner using the half exposure technology with a greytone mask for the photolithography for creating an island of non-crystalline silicon film. As a result, a plurality non-crystalline silicon layers having varying thicknesses is obtained. Then, these non-crystalline silicon layers are irradiated with a laser beam to crystallize the regions of the non-crystalline silicon layers which have been made thinner (the source and drain regions of the photo sensor TFT and the region to become the active region of the switching TFT), while the region that has not been made thinner (the region to become the channel region of the photo sensor TFT) remains non-crystalline.